1. Field of the Invention
The invention relates to a measuring system for the optical measurement of an optical imaging system which is provided to image a pattern arranged in an object surface of the imaging system into an image surface of the imaging system, the imaging system being designed as an immersion system for imaging with the aid of an immersion liquid arranged on at least one of the object-side and the image-side of the imaging system
2. Description of the Related Prior Art
In many areas in technology and research, optical imaging systems are used, on which increasingly high requirements are placed with respect to their imaging quality. One example is the photolithographic production of semiconductor components and other finely structured components, in which, with the aid of high power projection objectives, at operating wavelengths in the deep ultraviolet range (DUV), structures in the submicrometre range can be produced. Because of the complicated optical structure of such imaging systems with a large number of optical components, it is necessary, both for the adjustment of the original production and also during any necessary maintenance work, to measure the imaging systems with regard to image defects which occur. The accuracy of the measuring systems and methods used for the testing is in this case normally matched to the requirements on the imaging accuracy of the imaging systems.
Use is frequently made of measuring systems which comprise an object-side structure carrier having an object-side measuring structure, to be arranged on the object side of the imaging system, and an image-side structure carrier having an image-side measuring structure, to be arranged on the image side of the imaging system, the object-side measuring structure and the image-side measuring structure being matched to each other in such a way that, when the object-side measuring structure is imaged onto the image-side measuring structure with the aid of the imaging system, a superposition pattern is produced. A detector for the locally resolving acquisition of the superposition pattern and an evaluation device connected thereto for determining at least one imaging parameter describing the imaging quality of the imaging system from the superposition pattern are used to produce measured values which permit conclusions to be drawn about the imaging quality that can be achieved with the imaging system and from which predefinitions for an optimization of the imaging system can be derived.
An apparatus operating in the manner of a shearing interferometer for wave front acquisition is described in German patent application DE 101 09 929 (corresponding to patent application US 2002/0001088 A1 from the applicant). In this measuring system, an object-side structure carrier to be illuminated with incoherent light is arranged in the object surface of the imaging system to be tested and has the task of setting a predefined level of coherence of the measuring radiation passing through the imaging system and is therefore also designated a coherence mask. The coherence mask provided can be, for example, a transparent carrier fabricated from quartz glass, on which the object-side measuring structure is applied in the form of a two-dimensionally structured coating with chromium. Arranged in the image surface of the imaging system is an image-side structure carrier having an image-side measuring structure acting as a diffraction grating. This can likewise be formed by a chromium layer, which is applied to a transparent quartz glass carrier. As a result of the superposition of the waves produced by diffraction, a superposition pattern in the form of an interferogram (shearogram) is produced, which is acquired with the aid of the locally resolving detector connected downstream and is then evaluated. Similar measuring systems are disclosed by EP 1 231 517 A1.
The meaningfulness of the measuring method is particularly high if, for this wave front measurement, use is made of the same radiation which is also used in the intended use of the imaging system, for example ultraviolet light from the DUV range. For this purpose, the measuring system can be integrated into a microlithographic projection exposure system, so that, for the measurement of a projection objective, the same illumination system which is also used during production operation can be used. In the case of external measurement, independent measuring systems with illumination systems which are of the same type or of a similar type can be used.
Other interferometers can likewise be used, for example interferometers of the Ronchi type or Twyman-Green type. Furthermore, point diffraction interferometers (PDI) are also common as measuring systems. Examples are shown in the documents U.S. Pat. No. 6,307,635 B1 or WO 02/42728 A1. An object-side measuring structure to be arranged on the object side of the imaging system has a pinhole, which is arranged on an object-side structure carrier and is used to generate a spherical wave from the illuminating radiation. On the image side of the imaging system there is arranged a further pinhole structure in which, in addition to a pinhole, a second, larger opening is provided to allow a specimen wave to pass through freely. As a further measuring structure which, in particular, is arranged between object surface and imaging objective or between imaging objective and image surface, a diffraction grating serving as a beam splitter is provided. The fine structures of the pinhole masks and of the diffraction grating can be formed by microstructured coatings on transparent carriers.
Other measuring systems and methods, in particular for measuring the distortion of optical imaging systems, are based on using the Moiré effect. In this case, an object grating is arranged in the object surface of the imaging system to be tested and, for example, comprises a large number of parallel, non-transparent lines which form the object-side measuring structure. In the image plane there is arranged an image-side measuring structure which is similar to the object-side measuring structure, the object-side measuring structure and the image-side measuring structure being matched to each other, while taking account of the imaging scale of the imaging system, such that when the object-side measuring structure is imaged onto the image-side measuring structure, a superposition pattern in the form of a Moiré pattern with Moiré fringes is produced. From the intensity distribution of the fringe pattern, which is acquired with a locally resolving detector, imaging parameters can be determined, for example for the distortion. Moiré methods are known, for example, from the patents U.S. Pat. No. 5,769,954, U.S. Pat. No. 5,973,773 or EP 0 418 054.
Furthermore, it may be necessary, in addition to the above-mentioned measuring structures but also independently of the latter, to fit a diaphragm structure in the image plane, which is used for the purpose of keeping undesired contributions of the image field away from detection elements of the measuring system, in order to minimize the error contribution from stray light. This diaphragm structure is preferably likewise formed by a transparent substrate coated with structured chromium.
In the case of projection objectives for microlithography, attempts are made to increase the resolving power by various measures to such an extent that finer and finer structures, for example of the order of magnitude of 100 nm or less, can be produced. For this purpose, firstly the image-side numerical apertures (NA) of the projection objectives are increased to values in the region of NA=0.8 or above. Secondly, shorter and shorter wavelengths of ultraviolet light are used, for example the laser wavelengths 193 nm or 157 nm.
There are approaches to improving the achievable resolution by an immersion liquid with a high refractive index being introduced into the space between the last image-side optical element of the projection objective and the substrate to be exposed. This technique is designated immersion lithography; the projection objectives suitable for this purpose are designated immersion objectives. The theoretical advantages of immersion lithography lie in the possible increase in the numerical aperture to values NA>1 and thus a possible increase in the resolving power or an improved depth of focus. This can be achieved with an unchanged vacuum wavelength, so that techniques of light generation, of the selection of optical materials, of coating technology, etc., established for the corresponding wavelength, can be transferred largely unchanged. The use of immersion media is in addition a precondition for the use of projection objectives having extremely high numerical apertures in the region of NA=1 or above.
For 193 nm, extremely pure water with a refractive index nl=1.437 stands out as a suitable immersion liquid. For 157 nm, at present immersion liquids based on perfluoropolyethers (PFPE) are favored. One immersion liquid tested has a refractive index nl=1.37 at 157 nm (see the article “Immersion Lithography at 157 nm”, by M. Switkes and M. Rothschild, J. Vac. Sci. Technol. B 19(6), November/December 2001, pages 1 ff.)
Since, for meaningful measured results, it is advantageous to carry out the measurement of the imaging system under conditions which are similar to the conditions during the intended use or are identical to these, attempts have already been made to adapt the measurement techniques established for dry systems to the measurement of immersion objectives. In the patent application DE 102 61 775.9 from the applicant, which has not yet been published, measuring systems are presented which are adapted for the measurement of immersion systems in that means forming fluid chambers are provided, in order for example to delimit an interspace between an object-side structure carrier and the imaging system and/or the interspace between an image-side structure carrier and the imaging system in such a way that an immersion liquid chamber is produced in which the immersion liquid can be held, at least for the period of the measurement. The disclosure content of this patent application is incorporated by reference in the present description.
According to observations by the inventors, during measurements under immersion conditions, it is possible for a gradual worsening of the measuring accuracy to occur.